And the number is growing, much to the alarm of pediatricians and state health officials who fear diseases that were once nearly eradicated may proliferate as a result.

Over the past decade, "personal belief exemptions" have tripled. Signed by parents, PBEs allow children to enter school missing some or all vaccines. Statewide, more than 2 percent of kindergartners have such exemptions.

With a 9.5 percent PBE rate -- more than four times the state average -- Santa Cruz County is close to the epicenter of this often heated and emotional debate. In the county's northern part, the parents of about 17 percent of entering kindergartners signed exemptions last fall -- one of California's highest rates.

State experts say that as long as 95 percent of a population is immunized, "herd immunity" keeps contagious diseases from spreading. But vaccine refusal tends to concentrate in geographical areas like northern Santa Cruz County, and that creates risk.

While most Bay Area counties have exemption rates on par with the state average, certain schools within them have high rates of vaccine refusal, including Contra Costa County's East Bay Waldorf, which closed temporarily in 2008 after at least 16 children came down with whooping cough. Last year, that school reported a 75 percent exemption rate.

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"If they were scattered fairly evenly around the state, the implications would be much less concerning," said Dr. Rob Schechter, medical officer with the immunization branch of the state Department of Public Health, which collects the statistics on student immunizations. "The fact that they are concentrated in communities, social networks and schools with much higher rates -- that allows disease to spread much more rapidly."

Among those most at risk are small infants and medically fragile individuals who cannot be vaccinated and could be infected by unvaccinated family members or friends. Even a small percentage of those who have been vaccinated lack full immunity, doctors say.

Despite knowing these risks, many parents grapple with whether to vaccinate.

Lucia Paxton, a 39-year-old Santa Cruz resident, plans to sign a PBE form when her 10-year-old daughter enters middle school, just as she did when the girl entered kindergarten. Paxton and her husband decided against vaccination after reading six books on the subject and talking with a relative whose child had a bad reaction to a vaccine. She rejects the notion that this is a counterculture trend led by "irresponsible" or "hippie-loving" parents.

"Your child's health is not a trend," she said. "Every parent wants their kids to be healthy and to have a good life."

A few years ago, their normally healthy daughter -- whom they home school -- came down with whooping cough.

She revisits her decision every year, and says she's considering getting her daughter the chickenpox vaccine, since the disease can be dangerous for teenagers.

Parents and doctors on all sides of the debate point to vast quantities of conflicting information on the Internet. Despite the recent discrediting of a 1998 article in the British medical journal Lancet linking vaccines and autism, fears of a connection still linger on the Web. Added to the mix is a widespread mistrust of pharmaceutical companies and the government, as well as the disappearance of many once-feared diseases. All this makes the threat of vaccines seem scarier to some than the threat of the ailments they've largely helped eliminate.

Many local doctors are watching with concern a measles outbreak in Europe, where vaccination rates have plummeted in recent years.

This follows last year's whooping cough outbreak -- the worst since 1947 -- which resulted in more than 9,100 reported cases and 10 infant deaths. In its aftermath, California is undertaking the largest "mandatory" vaccination campaign in recent memory. All seventh- through 12th-graders are required to have a whooping cough booster shot within 30 days of starting school.

That is, unless they file a personal belief exemption.

In 2005 and 2006, the Santa Cruz County Health Services Agency surveyed families who had signed PBEs. Most responders were well-educated and relatively affluent. They tended to agree that vaccines were important for preventing diseases, but also thought vaccines contained harmful substances and may cause autism.

"They're usually great parents," said Dr. Kristina Mutén, a physician at Santa Cruz Women's Health Center. "They're usually breast-feeding and feeding their kids organic food and don't let their kids watch a lot of TV."

But those parents aren't always getting the most accurate information, Mutén says. Local pediatricians echo state and national health organizations, telling patients that study after study shows vaccination is safe.

But some of the county's chiropractors, Chinese medicine doctors and other alternative medical providers oppose some or all vaccines.

Dr. Tom Cowan, a San Francisco doctor who specializes in anthroposophical medicine, worries about the toxic ingredients in vaccines and about how vaccines affect children's developing immune systems.

"The earlier and more often you vaccinate children, the more often you get allergy, asthma and eczema," he said. "The fact of the matter is vaccines don't eliminate diseases, they change diseases."

Currently, some 20 states, including California, allow parents to skip vaccinations on the basis of personal belief. Some of these states require a signature from a child's pediatrician before granting a vaccine exemption. California does not. Vaccine advocates have discussed proposing that policy here; vaccine critics roundly reject it.

Before she gave birth to her daughter, Navah, Rebecca Bishop read vaccination literature, talked to her friends and quizzed her doctor. Bishop, 36, a medical transcriptionist from Santa Cruz, knows polio and measles are scary diseases. But stories about the unintended effects of vaccines frightened her.

In the end, she and her husband decided to vaccinate. But they don't judge neighbors who decide otherwise.

"It's really hard to know who to believe or what to believe right now," she said.

Jocelyn Wiener reports for the CHCF Center for Health Reporting, an independent news organization devoted to reporting about health care issues that concern Californians. Headquartered at the University of Southern California's Annenberg School for Communication & Journalism, the center is funded by the nonprofit, nonpartisan California HealthCare Foundation.

VACCINE? NO, THANKS

The Center for Health Reporting analyzed state Department of Public Health data on vaccination compliance rates for kindergartners in the state's 58 counties. The following are personal belief exemption rankings and rates for select counties in 2010.